It is a long-standing objective of color photographic origination materials to maximize the overall response to light while maintaining the lowest possible granularity. Increased photographic sensitivity to light (commonly referred to as photographic speed) allows for improved images captured under low light conditions or improved details in the shadowed regions of the image. In general, the overall light sensitivity provided by the light sensitive silver halide emulsions in such systems is determined by the grain size of the emulsions. Larger emulsions capture more light. For tabular emulsions, the photographic speed would be proportional to the projected area (or diameter, d squared)-see for example James "The Theory of the Photographic Process" 4.sup.th ed. p 105 (where the photographic speed is measured as some threshold density value). In color photographic elements, upon development, the captured light is ultimately converted into dye deposits which constitute the reproduced image. However, the granularity expressed by these dye deposits is directly proportional to the grain size of the silver halide emulsion . Again for tabular emulsions, granularity is generally proportional to the square root of the grain area ie proportional to the grain diameter, d (James "The Theory of the Photographic Process" 4.sup.th ed. p 625).
Thus, larger silver halide emulsion grains have higher sensitivity to light (proportional to d.sup.2) but also lead to higher granularity in the reproduced image (proportional to d). It has been a long-standing problem to provide materials which maximize the response to light of a silver halide emulsion for any given grain size.
The problem of maximizing response of the emulsion grain to light is particularly important for the blue sensitive emulsions of high speed materials, since standard scene illuminants are at least somewhat deficient in blue light. As a result, 3D AgBrI emulsions with light absorption enhanced by high iodide content are generally employed in the fast yellow emulsion layer of the highest speed color photographic films. Unfortunately, these large fast yellow 3D emulsions scatter light in a very diffuse (sideways) manner and thereby compromise the acutance of underlying light sensitive layers. Tabular grains as fast yellow emulsions offer advantages for acutance of underlying layers due to the specular manner (forward direction) in which they scatter light but up until now have been deficient for adequate speed/granularity. Here our usage of the term acutance is that generally offered in standard reference works such as James "The Theory of the Photographic Process" 4.sup.th ed. Pp 602-607.
It is of particular interest to find solutions to this problem for large emulsions with the potential for providing high speed (preferably ISO 400 or greater) color photographic materials. Such high speed materials have a number of potential applications. They are particularly valuable for use in cameras with zoom lenses and in single use cameras (also called "film with lens" units). Zoom lenses generally have smaller apertures (higher f-numbers) than comparable fixed focus lenses. Thus, zoom lenses, while giving increased flexibility in composition of a pictorial scene, deliver less light to the camera film plane. Use of high speed films allows the flexibility of zoom lenses while still preserving picture taking opportunities at low light levels. In single use cameras, lens focus is fixed. Here, high speed films allow use of a fixed aperture having a higher f-number, thus increasing the available depth of field, an important feature in a fixed focus camera. For single use cameras with flash, higher film speed allows pictures to be taken with a less energetic flash, enabling more economical manufacture of the single use unit.
A dramatic increase in photographic speeds in silver halide photography began with the introduction of tabular grain emulsions into silver halide photographic products in 1982. A tabular grain is one which has two parallel major faces that are clearly larger than any other crystal face and which has an aspect ratio of at least 2. Tabular grain emulsions are those in which tabular grains account for greater than 50 percent of total grain projected area. Kofron et al U.S. Pat. No. 4,439,520 illustrates the first chemically and spectrally sensitized high aspect ratio (average aspect ratio &gt;8) tabular grain emulsions. In their most commonly used form tabular grain emulsions contain tabular grains that have major faces lying in {111 } crystal lattice planes and contain greater than 50 mole percent bromide, based on silver. A summary of tabular grain emulsions is contained in Research Disclosure, Item 38957, I. Emulsion grains and their preparation, B. Grain morphology, particularly sub-paragraphs (1) and (3).
The use of cationic starch as a peptizer for the precipitation of high bromide {111} tabular grain emulsions is taught by Maskasky U.S. Pat. Nos. 5,604,085, 5,620,840, 5,667,955, 5,691,131, and 5,733,718. Oxidized cationic starches are advantageous in exhibiting lower levels of viscosity than gelatino-peptizers. This facilitates mixing. Under comparable levels of chemical sensitization higher photographic speeds can be realized using cationic starch peptizers. Alternatively, speeds equal to those obtained using gelatino-peptizers can be achieved at lower precipitation and/or sensitization temperatures, thereby avoiding unwanted grain ripening.
To increase the speed of silver halide emulsions independent of spectral sensitization, the grain surfaces are treated with chemical sensitizers. A summary of chemical sensitizers is provided by Research Disclosure, Item 38957, cited above, IV. Chemical sensitization.
It has been recently recognized that a further enhancement in photographic speed can be realized by associating with the silver halide grain surfaces a fragmentable electron donating (FED) sensitizer. While no proof of the mechanism of FED sensitization has yet been generated, one plausible explanation is as follows: When, as noted above, photon capture within a grain results in electron promotion from a valence shell to a conduction energy band, a common loss factor is recombination. That is, the promoted electron simply returns to a hole in the valence shell, created by promotion to the conduction band of the same or another electron. When recombination occurs, the energy of the captured photon is dissipated without contributing to latent image formation. It is believed that the FED sensitizer reduces recombination by donating an electron to fill the hole created by photon capture. Thus, fewer conduction band electrons return to hole sites in valence bands and more electrons are available to participate in latent image formation.
When the FED sensitizer donates an electron to a silver halide grain, it fragments, creating a cation and a free radical. The free radical is a single atom or compound that contains an unpaired valence shell electron and is for that reason highly unstable. If the oxidation potential of the free radical is equal to or more negative than -0.7 volt, the free radical immediately upon formation injects a second electron into the grain to eliminate its unpaired valence shell electron. When the free radical also donates an electron to the grain, it is apparent that absorption of a single photon in the grain has promoted an electron to the conduction band, stimulated the FED sensitizer to donate an electron to file the hole left behind by the promoted electron, thereby reducing hole-electron recombination, and injected a second electron. Thus, the FED sensitizer contributes one or two electrons to the silver grain that contribute directly or indirectly to latent image formation.
FED sensitizers and their utilization for increasing photographic speed are disclosed in U.S. Pat. Nos. 5,747,235, 5,747,236, 5,994,051, and 6,010,841, and published European Patent Applications 893,731 and 893,732.
When silver halide grains are developed, the light exposed (as opposed to the non-exposed) silver halide grains are selectively reduced with a developing agent. During this reaction silver halide is reduced to silver, and the developing agent is oxidized. When it is desired to form a dye image, the developing agent is usually chosen to be a color developing agent, which is a developing agent that, following oxidization, reacts to complete an image dye chromophore. The most common route to image dye formation is the reaction of an image dye-forming coupler with apara-phenylenediamine color developing agent, which is apara-phenylenediamine in which at least one of the amine groups is unsubstituted. Dye chromophore formation occurs when one or two quinonediimine molecules (each of which requires two molecules of oxidized para-phenylenediamine color developing agent to produce) reacts with the image dye-forming coupler. When an image dye-forming coupler requires two quinonediimine molecules to form an image dye molecule, the image dye-forming coupler is said to be a four equivalent coupler, since four molecules of color developing agent must be oxidized to result in each molecule of image dye. Two equivalent coupler image dye-forming couplers are those that spontaneously split off an anionic (e.g., halogen) or low pKa leaving group (e.g., phenol or heterocycle) under the conditions of development and therefore react with a single quinonediimine molecule to form an image dye molecule. These mechanisms of image dye formation are textbook knowledge, as illustrated by the Color Photography topic in The Kirk-Othmer Encyclopedia of Chemical Technology, John Wiley and Sons, New York, 1993; Vol. 6.
Since the molar ratio of image dye produced to developed silver is lower when a four equivalent image dye-forming coupler is employed than when a two equivalent image dye-forming coupler is employed and since the photographic speeds of color photographic elements are compared by measuring the exposure difference required to reach a reference image dye density, it is apparent that otherwise comparable color photographic elements containing two equivalent image dye-forming couplers exhibit higher imaging speeds than those that contain four equivalent image dye-forming couplers. This recognition led to investigation of one equivalent image dye-forming couplers. One equivalent image dye-forming couplers are similar to two equivalent image dye-forming couplers in that only one quinonediimine molecule is required to form an image dye molecule. One equivalent couplers differ from two equivalent couplers in that the leaving group that is split off prior to coupling itself supplies a molecule of image dye which is in addition to the molecule of image dye produced by coupling. Hence, reduction of two molecules of silver halide to silver produces two molecules of oxidized para-phenylenediamine color developing, which produce one molecule of quinonediimine that reacts with a one equivalent coupler to produce two image dye molecules. Hence, in theory there is a one to one molar ratio of developed silver to image dye. The unique requirements imposed by dye chromophore containing leaving groups in one equivalent image dye-forming couplers have limited their application, with two and four equivalent structures forming the overwhelming majority of image dye-forming couplers. One equivalent image dye-forming couplers are described in Mooberry et al U.S. Pat. Nos. 4,840,884, 5,447,819 and 5,457,004.